Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Some touch sensor panels can operate based on mutual capacitance. Mutual capacitance refers to the ability of two conducting drive and sense surfaces, arranged closely together but not directly coupled, to store a charge when a voltage is applied across them. In mutual capacitance touch sensor panels, for example, each of a number of sensors or pixels (representing a particular location on the touch screen) can be formed from opposing conducting elements arranged closely together. A particular sensor can be formed between a driving element (e.g. an arrangement of conductive drive traces formed as “fingers”) coupled to a drive line, and a sensing element (e.g. an arrangement of conductive sense traces formed as fingers) coupled to a sense line. When a stimulation source stimulates the drive line with a voltage at a particular frequency, the stimulus essentially injects a charge across the drive and sense elements due to their mutual capacitance. A sense or charge amplifier can sense the charge injected into the sensing element via the sense line. Further, the sensing element can be arranged so that a touch by a user's finger or other object at that particular sensor location can form a capacitance pathway between the driving element and earth ground through the user's body, causing some of the driving element's charge to escape to ground and reducing the charge coupled across to the sense element. The touch sensor panel can therefore detect touch based on changes to the level of charge sensed on the sensing element. A higher level of charge can indicate no touch on the panel, while a lower level of charge can indicate a touch on the panel.
As common in electronic devices, a certain amount of noise or electrical interference can exist in the touch sensor panel. Such noise can be common to the panel's conducting parts relative to the device ground, and can be referred to as common mode noise. Common mode noise in touch sensor panels can be coupled along paths that have parasitic or stray capacitances, such as between the driving and sensing elements and between the drive and sense lines. Because common mode noise tends to be relatively small and evenly distributed across the panel, it provides a relatively minor impact, if any, on the operation of the panel. However, a larger noise source that is not evenly distributed across the panel can impact the operation of the touch sensor panel.